Described herein are an ultrasonic surgical handpiece with the energy initiator circuit to begin and maintain the vibrations and linear dynamics of the elongate tapered pipe tool. The flue is coaxially about the tool.
U.S. Pat. No. 4,223,676 discloses an ultrasonic surgical handpiece having a longitudinally vibrating cutting tool ending in a tip with a flue to surround the vibrating tool. The disclosure of '676 is incorporated herein by reference and made a part of this disclosure since the surgical instrument therein is substantially refined and improved by that disclosed and claimed herein. Handpieces made in accordance with the teachings of the '676 patent have tools that were less than four inches long and even with extenders less than seven inches long. Heretofore the need for a tool that was greater than seven inches was unsatisfied and the flue and oscillating electronics needed to serve with such an elongate tool were undeveloped.
U.S. Pat. No. 4,747,820 owned by the assignee in this present disclosure has a rigid plastic flue constructed so that the body thereof may be rotated relative to a flue adapter, i.e., the piece that attaches to the distal end of the handpiece. The body and adapter are two separately made rigid plastic parts for allowing the relative rotary motion therebetween so that alignment may be achieved particularly where there is a curve in the flue body to accommodate a curved tool. An alternate flue of flexible polymer with supporting ribs is shown and described as received on a rigid adapter with an appropriate conjugating configuration.
While it might seem logical that an elongate tool requires a longer flue to supply irrigation and cooling fluid to the preaspiration holes near the tip of the elongate tool, many unsolved technicalities need resolutions which go beyond the mere extension of the length of the components. With increase in length the structure of the flue needs improvement to have the requisite strength and to provide the needed cooling flow to cover all of the increased size of the elongate tool. The flue disclosed herein is preferably of a soft polymer so that it will not injure the patient and in order to seal to the hand piece in a gasket like easily used way. Soft polymers such as silicon rubber have very little beam or bending strength and consequently, sag without support from the elongate tool or even with the elongate tool disposed coaxially therewithin the flue could lean under the force of gravity so as to be against the elongate tool, thus damping the tool vibrations. Circumferential or surround coolant is thus hampered by any off center disposition of the flue and the need to maintain concentricity between the flue and the elongate tool remains an unresolved problem.
In addition the dispersion of the irrigation fluid so as to equally access all side and areas of the elongate tool is a problem relative to length. The fluid flow tends to be influenced by gravity as well as the flue deflection. That is, the distance from the support or supply to the distal end increases the problem. Uniform circumferential distribution is difficult with a long flue.
To initiate the vibration in an elongate tip the electronic oscillating circuitry requires particular attention to avoid start up transient overshoots causing the electromechanical oscillating system to equilibrate (or lock up) on an unwanted adjacent resonance. Said adjacent unwanted resonances are present, in particular, due to a long mechanical vibrator in that it has a lower fundamental resonance. The harmonics are integral multiples of the fundamental resulting in a closer spacing, in the frequency domain (of harmonics, or poles), as a function of vibrator length. For example, U.S. Pat. No. 4,587,958 has a starting impulse reducing circuit to prevent transient current from over driving the power amplifier by maintaining the waveform at a predetermined flat amount instead of an initiating spike or overshoot resulting during start up transient conditions. While it is recognized that peak current waveform suppression is useful to prevent spikes that can be two or more times greater that required, a circuit to control the rate of power application to an operational transconductance amplifier is not disclosed. It is now appreciated that the peak initiating current is not the only factor affecting amplifier performance. For consistent operating characteristics after starting and under loads the rate at which power is supplied to the amplifier in an oscillating circuit must be specifically controlled in a manner not disclosed in '958. Normal variations in the response characteristics of amplifiers can greatly influence the manner in which any specific amplifier of a particular type will respond to input power. Reduction of such variations and the uniform responses are desired.
A difficulty with merely applying power to the electronic circuitry that drives a mechanical reverberating system results from the tuning of the resonance frequency of the mechanical structure designed to vibrate in harmony with the spring constant of the system. An electrical driving circuit is inherently flexible and can oscillate at several frequencies more readily than any mechanical system. Consequently, the driving impetus of the electronic circuit can excite the mechanical structure at different frequencies than the intended resonance frequency. There are many mechanical resonant modes of acoustic vibrators that are independant of drive afforded by the electronics. When that occurs, the mechanical system will vibrate in a manner that does not produce useful displacement and the energy output from the electrical oscillator is wasted. Filtering can be used to assure that the driving frequency is identical with the harmonic of the mechanical system but that requires a complicated circuit design and can not be retrofitted to an existing oscillating circuit. A way to control the initiation and maintenance of the oscillation so that the operating frequency is at the preferred resonant frequency of the mechanical system is required.